The invention relates to an electromagnetic wave absorbent wherein magnetic powders are dispersed in an insulative resin as a bonding agent, and a method for producing magnetic powders for the electromagnetic wave absorbent.
For making functions of electronic machinery or communication apparatus stable, the electromagnetic wave absorbent is used in order to absorb electric waves to be external disturbance outside of the apparatus or electric waves escaping from the interior thereof for preventing noises or hindrance of electric waves.
Related art electromagnetic wave absorbents include irregular magnetic powders such as spinel or hexagonal ferrite sintered substances, which are dispersed in an insulative resin as a bonding agent.
Main applications for the electromagnetic wave absorbent include mobile communication machinery and other devices using a frequency band from para-microwave to microwave, such as portable telephones or PHS (personal handy-phone system) or casings of machinery.
In the electromagnetic wave absorbent, material parameters based on the electromagnetic wave absorbing properties have a complex dielectric constant and a complex permeability in a high frequency, and in the electromagnetic wave absorbent using the magnetic powders, a magnetic loss portion xcexcxe2x80x3 being an imaginary number component of the complex permeability xcexc=xcexcxe2x80x2xe2x88x92jxcexcxe2x80x3 plays a role in the electromagnetic wave absorbing properties.
The spinel ferrite based material has in general the complex permeability as shown in FIG. 4A. That is, when a frequency f increases a certain value, a real number xcexcxe2x80x2 of the permeability xcexc having been almost constant at that time rapidly goes down, and xcexcxe2x80x3 takes a maximal value in a resonance frequency fr being a higher frequency zone than xcexcxe2x80x2. The larger the maximal value of this xcexcxe2x80x3 is, the larger the energy loss generates, and the good electromagnetic absorbing properties are shown.
However, as seen in FIG. 4B, the higher resonance frequency (ferrite A less than ferrite B less than ferrite C) the spinel ferrite based material has, the smaller maximal value xcexcxe2x80x3 has. Therefore, a high permeability cannot be obtained in the high frequency particularly in such as a GHz zone, and therefore a good electromagnetic wave absorbing effect cannot be expected.
This is called as xe2x80x9csnoek""s critical linexe2x80x9d shown with a two-dotted line in the same, and a product of the resonance frequency and the permeability is constant in a formula (1).
[Formula 1:]                    fr        =                              γ                          3              ⁢                              πμ                0                                              ⁢          Is                                    (        1        )            
(In the formula, fr is a resonance frequency, xcexcxe2x80x2 is a real number, xcex3 is gyromagnetic constant, xcexc0 is a permeability of vacuum, and Is is saturation magnetization.)
In contrast, since the hexagonal ferrite sintered substance has a small magnetic anisotropy of an in-plane, the permeability is large. Moreover, the anisotropic energy is large to direct magnetization in a plane-orthogonal direction. Therefore, the resonance occurs at a higher frequency than that of the spinel ferrite sintered substance.
Namely, in the hexagonal ferrite sintered substance, the product of the resonance frequency and the permeability is expressed with a formula (2).
[Formula 2]:                               fr          ⁡                      (                                          μ                xe2x80x2                            -              1                        )                          =                                            γ              ⁢                              xe2x80x83                            ⁢              Is                                      3              ⁢                              πμ                0                                              ⁢                                                    H                A2                                            H                A1                                                                        (        2        )            
(In the formula, fr is resonance frequency, xcexcxe2x80x2 is real number, xcex3 is gyromagnetic constant, xcexc0 is permeability of vacuum, Is is saturation magnetization, HA1 is the magnetic anisotropy for directing the magnetic moment in the in-plane direction, and HA2 is the magnetic anisotropy for directing the magnetic moment in the plane-orthogonal direction.) Since HA2/HA1 in the formula is 1 or more, the high permeability can be maintained until a high frequency band exceeding xe2x80x9csnoek""s critical linexe2x80x9d.
However, the saturation magnetization of the hexagonal ferrite is around 0.5 T, and so the above-mentioned effect has been limited.
Therefore, the magnetic powders, which comprise a metallic soft magnetic material being a thickness around xe2x80x9cskin depthxe2x80x9d and being a flat shape of an aspect ratio (diameter/thickness) being 10 or higher, have been recognized as a material having a large magnetic loss portion xcexcxe2x80x3, which show a good electromagnetic wave absorption.
The thickness of xe2x80x9cskin depthxe2x80x9d is expressed with a formula (3).
[Formula 3:]                              (                      skin            ⁢                          xe2x80x83                        ⁢            depth                    )                =                              ρ                          π              ⁢                              xe2x80x83                            ⁢              f              ⁢                              xe2x80x83                            ⁢              μ                                                          (        3        )            
(p: electric resistivity, xcexc: magnetic permeability, f: frequency).
However, even if flat magnetic powders are used, the electromagnetic wave absorbent having an enough absorption effect is not always obtained in the present situation.
Therefore, in the related art, the demand for the high electromagnetic wave absorbing effect has been satisfied by increasing the rate of magnetic powders in the electromagnetic wave absorbent. However, the known electromagnetic wave absorbent has not complied with the recent demands for more intensively absorbing the electromagnetic wave in specific frequency bands depending on a further advanced higher output of the machinery.
As the ratio of the magnetic powders in the electromagnetic wave absorbent is increased, the ratio of the resin as the bonding agent is relatively less. The electromagnetic wave absorbent makes strength or formability less owing to the relative decrease of the ratio of the resin. Therefore, the increasing method of the rate of the magnetic powders has been limited.
For solving the above-mentioned problems, inventors carried out analyses on shapes and structure of magnetic powder, and found the following facts.
The present flat magnetic powders are generally produced by subjecting spherical raw powders made by, e.g., an atomizing process to mechanically breaking, elongating and tearing processes with a ball mill. In this method, even if the spherical raw powders are regulated almost in a uniform size, large dispersions occur in the sizes or shapes of produced magnetic powders, since strength to be loaded on the raw powders in subsequent breaking, elongating and tearing processes is different per each of powders. Therefore, the magnetic powders especially have large dispersions of plane shapes and thickness as to respective magnetic powders. Further, even though the sizes of the magnetic powders are classified and regulated in a certain range, dispersions of the plane shape and the thickness are large and the thickness of any portion of each magnetic powders are irregular. Therefore, the frequency properties are standardized between the magnetic powders, if the dispersions are large. In other words the frequency property does not have an acute peak of a specific frequency, but has a broad distribution over a wide frequency band. Therefore, the absorption effect of the magnetic powders is lowered in the specific frequency. Further, when the magnetic powders are dispersed into the resin, a waste of space occurs due to their irregularity in shape. Therefore, the known magnetic powders cannot obtain a high electromagnetic wave absorbing effect.
When the structure of the magnetic powders is considered, Nixe2x80x94Fe alloy shows a most excellent soft magnetic property among metallic soft magnetic materials. This alloy exhibits the highest soft magnetic property when it is of a solid solution under a non-equilibrium condition at room temperatures. However, as in Nixe2x80x94Fe alloy, an intermetallic compound Ni3Fe having the low soft magnetic property is under an equilibrium condition at room temperatures, the related art of the flat magnetic powder subjected to dissolution and cooling processes has a structure including the intermetallic compound. Therefore, from this structure, the high electromagnetic wave absorbing effect cannot be provided, either.
On the other hand, for solving the above-mentioned problems, it is proposed in JP-A-2001-60790 to use disc like magnetic powders having circular planes and uniform thickness. Detailed theory is described in the publication, but in generally the disc like magnetic powder comprising a metallic soft magnetic material, the ratio of HA2/HA1 is larger than the existing cases, where HA1 is the magnetic anisotropy for directing the magnetic moment in the in-plane direction, and HA1 is the magnetic anisotropy for directing the magnetic moment in the plane-orthogonal direction. Besides, the saturation magnetization of the metallic soft magnetic material is considerably higher than that of the hexagonal ferrite. Accordingly, it is presupposed that the disk like magnetic powder shows a higher permeability frequency zone than that of the present.
However, as described in the publication, ball-like raw powders formed by a water atomizing process are subjected to mechanically breaking, elongating and tearing processes into the magnetic powders in a flat shape by means of a ball mill, and although the ball-like raw powders are regulated almost uniformly in powder size, since strength to be loaded on the raw powders in subsequent breaking, elongating and tearing processes is different per each of the raw powders, large dispersions occur in the sizes or shapes of produced magnetic powders.
So, though classifying and regulating sizes in certain ranges, the magnetic powders especially have large dispersions of plane shapes and thickness as to respective powders, and besides they are irregular even inside of the same powders. If dispersions are large, frequency properties are standardized between or among powders. Namely, the frequency property does not have an acute peak with respect to specific frequencies, but has a broad distribution over a wide frequency zone and the absorption effect is lowered with respect to the specific frequencies. Further, being irregular in shape, when the magnetic powders are dispersed into a resin, their use is questionable in view of space consideration. Therefore, the known flat magnetic powder cannot obtain a high electromagnetic wave absorbing effect.
In view of the structure of the magnetic powders, a Nixe2x80x94Fe alloy called as permalloy shows a most excellent soft magnetic property among metallic soft magnetic materials. This alloy exhibits a highest property when it is of a solid solution under a non-equilibrium condition at room temperatures. But in the Nixe2x80x94Fe alloy, since an intermetallic compound having the low soft magnetic property being Ni3Fe is present under an equilibrium condition at room temperatures, the conventional magnetic powder having passed through a dissolution and a cooling has a structure including such an intermetallic compound. Therefore, seeing in the structure, the high electromagnetic wave absorbing effect cannot be provided, either.
In the above publication, studies have been made on a method of punching or etching magnetic film formed into desired dimensions or shapes by a vapor-phase growth process, such as a vacuum evaporation or a spattering process. Depending on the method, it is assumed that the magnetic powder having the plane shape regulated between respective powders and having the uniform thickness between respective magnetic powders and within one magnetic powder, may be produced.
However, seeing the magnetic powder from the structure, a processed structure remains in the magnetic powder, if the magnetic powder is punched. A corrosion structure remains in the magnetic powder, if the magnetic powder is etched. With this, the structure is disordered within the magnetic powders and the soft magnetic property goes down. Therefore, the high electromagnetic wave absorbing effect cannot be obtained.
If the film of the magnetic material is in advance pattern-formed by a vapor-phase growth process using a mask pattern, the problem of disorder in the structure is solved.
However, the thus pattern-formed film shows a tendency to be larger in thickness as going to a center and smaller as approaching a circumference near the mask pattern. Therefore, the thickness is irregular in the respective magnetic powders, and the electromagnetic wave absorbing effect goes down.
Further, the film formed through the vapor-phase growth process is difficult to separate from a mold. Thus, the film is easily deformed or damaged owing to stress when separating. Further, if dust by deformation or damage, which causes dispersions in the frequency property, are mixed into the powder, the absorbing effect for the electromagnetic wave of the specific frequency decreases more.
Moreover, a yield of the produced magnetic powder is around 30% of the used raw material in any cases when punching or etching the film formed through the vapor-phase growth process or when pattern-forming by use of the mask pattern. Further, an initial cost of an apparatus used in the vapor-phase growth process is considerably expensive. Therefore, there is a problem that a production cost including the initial cost is high.
It is an object of the invention to provide an electromagnetic wave absorbent, which includes magnetic powders showing the high permeability in the high frequency band such as the GHz zone, has an excellent effect in selectively, effectively and intensively absorbing an electromagnetic wave in specific frequency bands, and a method for producing magnetic powders for the electromagnetic wave absorbent.
The inventors made further investigations on the magnetic powders. As a result, they found that the magnetic powder should be produced by precipitating a magnetic film selectively in an electrode range by electroplating using a plating mold pattern-formed with the electrode range corresponding to the shape of the magnetic powder and an insulative range surrounding the periphery of the electrode range, and by peeling the film of magnetic material precipitated by the electroplating. Thus the inventors have accomplished the invention.
That is, the above-mentioned object can be achieved by an electromagnetic wave absorbent comprising: an insulative resin as a bonding agent; and a plurality of magnetic powders dispersed into the insulative resin, the magnetic powder being regular in the plane shape between the respective powders and being regular in thickness between the respective powders and within one magnetic powder.
The magnetic powder is produced by preparing a plating mold pattern-formed with an electrode range corresponding to the shape of the magnetic powder and an insulative range surrounding the periphery of the electrode range, precipitating a magnetic film, which has a plane shape corresponding to the shape of the magnetic powder, selectively in the electrode range through an electroplating with the plating mold while the electrode range being as a cathode, and by peeling the film from the plating mold.
The magnetic powder used in the electromagnetic wave absorbent according to the invention is made regular in the plane shape between powders in such a manner that the magnetic powder is formed in the plane shape in response to the shape of the electrode range of the plating mold by means of the electroplating as mentioned above. For instance, an area of the plane shape can be regulated in a range ofxc2x110% dispersion between powders. The plane shape of the magnetic powder is not limited to a specific shape. Preferably, the shapes are such as a circle or an ellipse without having corners, because these shapes limit influences of diamagnetism by a magnetization distribution to a minimum, and restrain dispersion of magnetic resonance frequency by shape anisotropy.
Further, depending on the electroplating, the film of magnetic material is precipitated on the electrode range in an almost uniform thickness. Moreover, in the electroplating, the thickness of the film of magnetic material can be strictly controlled to be a predetermined thickness by adjusting conditions as an electric current passing time, a current density and others. Therefore, it is possible with the method of the present invention to regulate the thickness of each magnetic powder within a range ofxc2x115% of the predetermined thickness. Likewise, it is possible to regulate the thickness of any portion of each magnetic powders within a range ofxc2x110% of the predetermine thickness. This regulation is made possible by the electroplating process employed by the present invention.
The film formed by the electroplating can be easily peeled from the plating mold in comparison with the vapor-phase growth process. Therefore, it is more difficult to deform and damage the film. With this, the magnetic powder can have the frequency property having an acute peak of the specific frequency, and when dispersing the magnetic powder into the resin, no waste of space occurs.
On the other hand, seeing from the structure, the film of magnetic material formed by the electroplating presents a state of the solid solution showing the highest soft magnetic property as mentioned above, if it is Nixe2x80x94Fe alloy. Besides, as it is previously pattern-formed, the structure is not disordered by punching or etching.
Accordingly, the electromagnetic wave absorbent of the invention using the magnetic powder, comparing with the related art, has an excellent effect in selectively, effectively and intensively absorbing electromagnetic waves in specific frequency band.
In order to heighten the permeability of the Nixe2x80x94Fe alloy, Ni and Fe are the solid solution in the Nixe2x80x94Fe alloy. Further, it is enumerated that the metallic structure has no lattice defect such as internal strain.
Therefore, the inventor made studies on thermal treatments of the magnetic powders produced by an electroplating for decreasing the lattice defect and accomplishing the higher permeability. Making experiments by varying temperature conditions of the thermal treatments, as a result, however, contrary to presumption, the higher temperatures the thermal treatments are performed, the lower the permeability becomes in the high frequency band.
It is found that when the thermal treatment is done at 300xc2x0 C. or higher, crystal grains grow to be coarse. That is, the average crystal grain diameter of the metallic soft magnetic material forming the magnetic powder are 100 nm or smaller without doing the thermal treatment. When the metallic soft magnetic material is heated at 300xc2x0 C. for 60 minutes, the crystal grain become coarsened until about 300 nm. When the metallic soft magnetic material is heated at 600xc2x0 C. for 60 minutes, the crystal grain become coarsened until about 2800 nm.
From these facts, it is found that in the flat magnetic powder, the smaller the average crystal grain diameters of the metallic soft magnetic material are, the larger the magnetic loss portion xcexcxe2x80x3 could be made.
Therefore, the inventor considers as follows. As shown in JP-A-2001-60790, if the HA2/HA1 has a large value, xcexcxe2x80x3 becomes high in the high frequency band.
As HA2 is determined owing to a shape of the magnetic powder, for more heightening xcexcxe2x80x3 of the same shape in the high frequency band than the present state, it is sufficient to make small the magnetic anisotropy HA1 when directing a magnetic moment in the in-plane.
In the case of the flat magnetic powder comprising the metallic soft magnetic material, the crystal grain is made fine to reduce the crystal grain diameter in order to make HA1, i.e., the crystal magnetic anisotropy, small.
If the crystal grain is made fine, the volumetric percentage of the grain boundary, which is being disorder in crystal arrangement, is high.
Therefore, the crystal magnetic anisotropy is small as a whole, and HA2/HA1 has the larger value than the present, thereby to make xcexcxe2x80x3 high in the high frequency band.
The inventor further studied the range of the average crystal grain diameter, and as a result, has found that the average crystal grain diameter is sufficiently 100 xcexcm or lower.
Accordingly, the electromagnetic wave absorbent of the present invention includes an insulative material as a bonding agent, and magnetic powders, which are much dispersed into the insulative resin. The magnetic powders have an average crystal grain diameter of 100 nm or smaller.
If it is considered to change in heating histories of the magnetic powders, for example, when melting and mixing the magnetic powder and resins under heating for producing the electromagnetic wave absorbent, and when forming the produced electromagnetic wave absorbent into desired shapes through the heat-forming, an average value of the crystal grains diameter is defined as the average crystal grain diameter immediately after producing the electromagnetic wave absorbent dispersed with the magnetic powder in the resin.